Which of these numbers is composite? ${5,\ 31,\ 43,\ 45,\ 53}$
A composite number is a number that has more than two factors (including 1 and itself). 5, 31, 43, and 53 each have only two factors. The factors of 45 are 1, 3, 5, 9, 15, and 45. Thus, 45 is the composite number.